Titanium base alloy



United States Patent 3,268,329 TITANIUM BASE ALLOY Harry W. Rosenberg, Henderson, Nev., assignor to Titanium Metals Corporation of America, New York, N.Y., a corporation of Delaware No Drawing. Filed Aug. 29, 1963, Ser. No. 305,511 4 Claims. (Cl. 75175.5)

This invention relates to titanium base alloys and more particularly to such an alloy characterized by a combination of high strength, toughness, weldability, corrosion resistance and low density.

Submarines capable of diving to extreme depths have been envisioned. Such a craft, however, poses an important problem insofar as hull construction is concerned. The metal or alloy used for hull construction must be of high strength to resist the tremendous hydraulic pressures encountered at extreme depths. Even so, the hull must be constructed of plate of substantial thickness and its density must be as low as possible so that its weight will not afiect, to a serious degree, the buoyancy produced by its enclosed volume. Toughness is desired in such an alloy to insure safety, and toughness at low temperature is particularly desirable to permit operation of the craft in polar regions. Weldability is necessary so that the hull may be fabricated as an integral unit. Resistance to corrosion, specifically in salt water, is obviously a desirable characteristic.

It is a principal object of this invention to provide an improved titanium base alloy. Another object of this invention is to provide a strong, tough, weldable, light and corrosion resistant alloy. These and other objects of this invention will be apparent from the following description thereof.

The alloy of this invention consists essentially of by weight 5.5 to 7.5% aluminum, from 1.7 to 2.3% tin, from 0.7 to 1.3% molybdenum, from 0.7 to 2.3% vanadin-m, balance titanium with incidental impurities. Such an alloy will possess an ultimate strength above 120,000 p.s.i., toughness above foot pounds, measured by the Charpy impact test at 80 F., low density and excellent corrosion resistance, particularly to salt water and good weldability.

A preferred composition within the ranges defined above consists essentially of about 6% aluminum, about 2% tin, about 1% molybdenum and about 1% vanadium with the balance titanium and incidental impurities. This alloy possesses, when heat treated, good ultimate strength of over 120,000 p.s.i. and a Charpy impact test of over 25 foot pounds at -80 F. showing good toughness. At the same time, the alloy is light, that is has low density, less than that of unalloyed titanium, and shows no corrosion when immersed in sea water for an indefinite period. Its weldability is good.

Another highly desirable composition consists essentially of about 7% aluminum, about 2% tin, about 1% molybdenum and about 1% vanadium, with the balance titanium and incidental impurities. This alloy is slightly lighter and a little stronger than the 6Al-2Sn-lMo-1V alloy but tends to be slightly less ductile and have lower toughness. Its other properties are essentially the same.

Still another highly desirable composition consists essentially of about 6% aluminum, about 2% tin, about 1% molybdenum and about 2% vanadium with the balance titanium and incidental impurities. This alloy is slight- 1y heavier than the 6Al-2Sn-lMo-1V alloy but is slightly stronger and tends to be slightly less tough. Its other properties are essentially the same.

The nature and amounts of alloying elements in the alloy of this invention are critical. Aluminum in amount between 5.5 and 7.5% provides lightness and strength. Less than 5.5% aluminum will not provide these properties to an optimum degree and more than 7.5 aluminum will tend to make the alloy brittle. About 2% tin, as employed in the alloy of this invention, further increases strength without loss of ductility. The aluminum and tin apparently act in combination to provide strength without brittleness and to produce an essentially all alpha type alloy. Less than about 2% tin will result in lowered strength and more than about 2% tin will increase strength somewhat but will result in disadvantageous loss of toughness, as well as resulting in higher density.

The amounts of molybdenum and vanadium defined above provide additional strength and toughness in the alloy and being beta stabilizers provide heat treatability. Less than 0.7% of each of molybdenum and vanadium will not provide these properties to an optimum degree while more than 1.3% molybdenum and more than 2.3% vanadium will be disadvantageous in providing more beta stibilizing element than is desirable, thus to an extent resulting in loss of the all alpha characteristics of the alloy. This will adversely affect weldability. In addition, both molybdenum and vanadium are heavier than titanium, particularly molybdenum, and more than a minimum amount to provide the properties required raises the density inordinately. Vanadium, being lighter, can be employed in slightly greater amount than molybdenum as recited but more than 2.3% of this element is not desirable for the reasons given above. I

Incidental impurities which may be present in the alloy of this invention will in general include the interstitials, oxygen, nitrogen and carbon as well as small or trace amounts of other elements which may be present in the titanium and other constituents from which the alloy is made. Such impurities should not in the aggregate exceed about 0.5% and should not individually or in combination exceed amounts which would significantly affect the characteristics or properties of the alloy as described. For example, oxygen and nitrogen are both known to exert an embrittling effect and the amounts of .these elements individually present should not exceed 0.2% and 0.15% respectively.

The alloy of this invention may be produced by a convenient method by which the titanium and alloying elements are melted together to form a substantially homogeneous composition. Preferably, titanium sponge of required purity is admixed with subdivided aluminum, tin, molybdenum and vanadium in proper amounts and the mixture compressed to form compacts. These compacts are welded together to form an electrode which is melted in a consumable electrode arc melting furnace to produce an ingot of alloy. The so-produced alloy ingot may be itself employed as an electrode in a subsequent remelting step to provide improved homogeneity in a final alloy ingot.

Table 1 below shows tensile strength and toughness (Charpy impact test at F.) for selected alloys of this invention. The values are for small double melted ingots of each alloy with tensile and impact specimens machined directly from the as-cast ingot. Tensile specimens; standard cylindrical specimens 3.00 inches total length with 1.250 inch reduction section, 0.250 inch diameter. Impact specimens; 0.452 inch round by 2.16 inches long with central 60 notch to diameter of 0.381 inch cooled to 80 F. before testing on a Sontag impact tester.

The values in Table 1 above show good strength, ductility and toughness in the as-cast condition, and this would indicate that similar properties would be obtained in welded areas.

Density of each of these allays is shown compared to that for pure titanium in Table 2 below:

Table 2 Alloy (Bal. Ti): Density, lbs/in. 6Al-2Sn-1Mo-1V 0.161

7Al-2Sn-1Mo-1V 0.160 6A1-2Sn-1M0-2V 0.162 Pure titanium 0.163

Corrosion resistance of these alloys in salt water (sea water) will correspond to that of pure titanium which is substantially immune to corrosion in this medium.

Tests made using a typical and preferred alloy composition, namely 6Al-2Sn-1Mo-1V, in the form: (1) of /2" plate as rolled, (2) of /2" plate as rolled and heat treated, (3) of 1" plate as rolled and annealed and also, (4) of /z" plate as welded are shown in Table 3.

Table 3 V Plate 1 Plate Plato Heat Treat Annealed Plate As Rolled ed 17501 1400F(1Hr) As Welded (Min) Air Cooled Air Cooled Ultimate Tensile Strength, p.s.i 140, 000 136, 000 120, 000 139, 000 Yield Strength, 0.2%

tiset, p.s.i 134, 000 125, 000 112, 000 *123, 000 Percent Red. in

Area 41 41 33 *35 Percent E1ongation 17 17 14 *15 Impact 80 F. ft.

lbs 2S. 1 31. 8 28. 3 27. 1

Values are average of specimens longitudinal and transverse to direction of plate rolling, except longitudinal direction any.

It is Well known in the metallurgical arts that high strength and toughness are not generally compatible. That is, strong alloys are relatively brittle while toughness is most often found in relatively lower strength alloys.

Titanium base alloys showing both high strength and toughness are rare because such strengthening is normally obtained by providing in the alloy significant amounts of both alpha and beta phases and such alloys exhibit poor toughness. However, I have found that when some beta phase titanium is present as a result of the inclusion of small and critical amounts of the combination of molybdenum and vanadium, both strength and toughness are enhanced and this unique eiiect appears to be obtained when aluminum and tin are employed as described as the alpha stabilizing elements.

Furthermore, the single phase alpha type titanium base alloys possess the best weldability characteristics whereas alpha-beta type alloys containing an appreciable amount of the beta phase are difiicult to Weld, if wel-dable at all. However, in the alloy of the present invention, the :beta phase is present only in :a small critical amount and weldability is substantially the same as that of comparable all alpha phase alloys.

Thus the alloy of the present invention possesses the unique and valuable combination of high strength, toughness particularly at low temperature, and Weldability and these properties are found in an alloy having a density similar to that of pure titanium and showing the superb corrosion resistance properties of titanium particularly during exposure in salt water. Such properties make it useful for applications Where its particular attributes are specifically desired such as, for example, in marine construction and especially for fabrication of submarine hulls.

I claim:

1. An alloy consisting essentially of by weight from 5.5 to 7.5% aluminum, from 1.5 to 2.5% tin, from 0.7 to 1.3% molybdenum, and from 0.7 to 2.3% vanadium, up to 0.5% of carbon, oxygen and nitrogen, but not to exceed 0.2% oxygen and 0.15% nitrogen with the balance titanium and incidental impurities, characterized by an ultimate tensile strength of over 120,000 p.s.i., by a toughness of over 25 foot pounds measured at 80 F. by the Charpy impact test, and by a substantially -all-alpha microstructure. I

2. An alloy consisting essentially of by weight about 6% aluminum, about 2% tin, about 1% molybdenum and about 1% vanadium with the balance titanium and incidental impurities, characterized by an ultimate tensile strength of over 120,000 p.s.i. by a toughness of over 25 foot pounds measured at 80 F. by the Charpy impact test, and by a substantially all-alpha microstructure.

3. An alloy consisting essentially of by weight about 6% aluminum, about 2% tin, about 1% molybdenum and about 2% vanadium with the balance titanium and incidental impurities, characterized by an ultimate tensile strength of over 120,000 p.s.i., by .a toughness of over 25 foot pounds measured at 80 F. by the Charpy impact test, and by a substantially all-alpha microstructure.

4. An alloy consisting essentially of by weight about 7% aluminum, about 2% tin, about 1% molybdenum and about 1% vanadium with the balance titanium and incidental impurities, characterized by an ultimate tensile strength of over 120,000 p.s.i., by a toughness of over 25 foot pounds measured at -80 F. by the Charpy impact test, and by a substantially all-alpha microstructu're.

References Cited by the Examiner UNITED STATES PATENTS 2,754,203 7/1956 Vordahl l75.5 2,893,864 7/1959 Harris 75175.5

FOREIGN PATENTS 525,344 5/ 1956 Canada.

HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

W. C. TOWNSEND, C. N. LOVELL,

Assistant Examiners. 

1. AN ALLOY CONSISTING ESSENTIALLY OF BY WEIGHT FROM 5.5 TO 7.5% ALUMINUM, FROM 1.5 TO 2.5% TIN, FROM 0.7 TO 1.3% MOLYBDENUM, AND FROM 0.7 TO 2.3% VANADIUM, UP TO 0.5% OF CARBON, OXYGEN AND NITROGEN, BUT NOT TO EXCEED 0.2% OXYGEN AND 0.15% NITROGEN WITH THE BALANCE TITANIUM AND INCIDENTAL IMPURITIES, CHARACTERIZED BY AN ULTIMATE TENSILE STRENGTH OF OVER 120,000 P.S.I., BY A TOUGHNESS OF OVER 25 FOOT POUNDS MEASURED AT -80*F. BY THE CHARPY IMPACT TEST, AND BY A SUBSTANTIALLY ALL-ALPHA MICROSTRUCTURE. 